Shisui has never really shown any notable feats, other than being one of the very few people to be named as “Turn-and-run”, meaning when you see him, your instant reaction is to turn and run. Now in overall Genjutsu POWER, Shisui takes the cake, Kotoamatsukami is an EXTREMELY over-powered jutsu and has pretty much no counter. And on top of that, Shisui is the only person that can use this ability every single day. The Hokage even knew this ability was very strong due to him relying on it to end the coup of the Uchiha, instead of having to kill them, of course Danzo didn’t like that and we all know how that worked out.
Now in overal practicality and how Shisui and Itachi uses Genjutsu, I’d have to give Itachi that one. He uses his Genjutsu in ways no one has thought of, even if you avoid his eyes, he simply has to point at you and BAM, Genjutsu. Plus, Tsukyomi doesn’t have a time limit, and Itachi could use this multiple times a day, as we saw in his fight with Naruto and Kakashi. Itachi was also extremely smart, and even used Genjutsu (albeit it was shisuis Kotoamatsukami) to break free of Edo Tensei, which as of yet, only Madara has been seen to pull off.
This is a 50/50 answer depending on how you phrase the question, in Genjutsu power, it goes to Shisui. Genjutsu usage and how they go about activating the Genjutsu, that goes to Itachi. I hope I helped in whatever way you needed it my dude.
Alright, let’s look at the evidence!